1. Field of the Invention
The invention relates to an active matrix type liquid crystal display device and a method of fabricating the same, and more particularly to an active matrix type liquid crystal display device which is capable of enhancing an aperture ratio and displaying images with high brightness without an increase in complexity in fabrication, and a method of fabricating such an active matrix type liquid crystal display device.
2. Description of the Related Art
In an active matrix type liquid crystal display device including a backlight source, a backlight intensity is usually enhanced for displaying images with a high brightness. However, this would consume much power, which does not satisfy small power consumption which is required in any display device.
In order to enhance a brightness in a liquid crystal display device including a backlight source, it is frequently carried out to enhance a light-transmission ratio. However, enhancement of a light-transmission ratio in a liquid crystal display device is equivalent to enhancement of a transmission ratio in a color filter which contributes to reduction in a light-transmission ratio. In order to enhance a transmission ratio in a color filter, a content ratio of pigment in a color filter is usually reduced, or a thickness of a color filter is reduced with a content ratio of pigment being kept as it is. However, if a transmission ratio in a color filter is enhanced by doing so, it would be necessary to control a color of backlight.
For the above-mentioned reason, it is usually carried out to enhance an aperture ratio of an opening in a substrate on which a thin film transistor is fabricated, in order to enhance a light-transmission ratio of a liquid crystal display device. Specifically, an area of each of pixels arranged in an opening is increased by reducing a wiring width and/or a size of a transistor, for instance.
FIG. 1 is a top plan view of a TFT substrate in a conventional liquid crystal display device including a backlight device.
In the illustrated liquid crystal display device, a plurality of scanning lines or gate lines 502 and a plurality of signal lines and drain lines 503 are formed on a transparent substrate in a matrix perpendicularly to each other. An area surrounded by each of the scanning lines 502 and each of the signal lines 503 defines a pixel area 504.
A gate electrode 505 is formed of a portion of each of the scanning lines 502 at a corner of the pixel area 504. On the gate electrode 505 is formed an island 506 comprised of a semiconductor layer, and on the island are formed a drain electrode 511 and a source electrode 512. The gate electrode 505, the island 506, the drain electrode 511 and the source electrode 512 constitute a thin film transistor (TFT) 514.
A transparent electrode 517 composed of indium tin oxide (ITO) is formed in the pixel area 504. The source electrode 512 is electrically connected to the transparent electrode 517 through a contact hole 516. The drain electrode 511 is formed integral with one of the signal lines 503.
The scanning lines 502, the signal lines 503 and the thin film transistor 514 are covered with a black matrix layer formed on an opposing substrate (not illustrated). An area other than the black matrix layer defines an aperture.
With reference to FIG. 1, a pixel area theoretically separated in a substrate on which the thin film transistor 514 is fabricated, specifically, a pixel area defined by a dotted line X is defined as a maximum aperture, and an area in which images are displayed by means of the transparent electrode 514, specifically, an area defined by a dotted line Y is defined as an actual aperture. An aperture ratio in the illustrated liquid crystal display device is defined as a ratio of an area of the actual aperture to an area of the maximum aperture, that is, Y/X.
Hence, in order to increase an aperture ratio without changing a size of each of the pixel areas used for displaying images, a width of wirings such as the scanning and signal lines 502 and 503 covered with the black matrix layer may be reduced. However, this causes an increase in a resistance of the wirings, preventing a high-rate operation of a liquid crystal display device.
If the thin film transistor 514 and the contact hole 516 were reduced in size without changing a size of a pixel area, characteristics of the thin film transistor 514 would be deteriorated with the result of degradation in displayed images. Thus, it is quite difficult to increase an aperture ratio merely by reducing a size of wirings and/or the thin film transistor 514.
With reduction in a size of a pixel in accordance with a requirement to displaying images at a higher density, a ratio of wirings and/or a thin film transistor to a maximum aperture becomes greater, resulting reduction in an aperture ratio. That is, displaying image at a high density causes reduction in a maximum aperture.
However, a size of wirings and a thin film transistor reaches its lower limitation. As a result, only an area of an actual aperture is reduced, and resultingly, an aperture ratio defined as a ratio of an area of the actual aperture to an area of the maximum aperture (Y/X) is reduced. Even so, if wirings, a thin film transistor and a contact hole were reduced in size in accordance with reduction in the maximum aperture, an aperture ratio remains unchanged. However, as mentioned earlier, a resistance of wirings would be increased, and characteristics of a thin film transistor would be deteriorated.
Japanese Unexamined Patent Publication No. 8-262495 (A) has suggested a liquid crystal display device for the purpose of enhancing an aperture ratio.
In the suggested liquid crystal display device, a thin film transistor is arranged in an area at which a scanning line or a gate line and a signal line or a drain line intersect with each other. By arranging a thin film transistor in such an area, it is no longer necessary to arrange a thin film transistor in a maximum aperture, ensuring enhancement of an aperture ratio.
In the suggested liquid crystal display device, a semiconductor layer such as an amorphous silicon layer is formed on a scanning line in the above-mentioned area, and then, source and drain electrodes are formed on the semiconductor layer to thereby fabricate a thin film transistor. Then, a signal line is formed so as to cover the thin film transistor therewith, and subsequently, a drain electrode is electrically connected to the signal line.
Thus, a process of fabricating the suggested liquid crystal display device includes two photolithography steps, that is, a first photolithography step of forming source and drain electrode, and a second photolithography step of forming a signal line.
In a conventional process of fabricating a liquid crystal display device, a photolithography step is carried out only once, because source and drain electrodes are formed concurrently with formation of a signal line. Thus, a process of fabricating the suggested liquid crystal display device has to carry out a photolithography step once greater than a conventional process of fabricating a liquid crystal display device, resulting in complexity in fabrication of a liquid crystal display device.
Japanese Unexamined Patent Publication No. 6-82832 (A) has suggested a liquid crystal display device including a TFT substrate comprised of an electrically insulating substrate, a plurality of gate lines formed on the substrate, a plurality of signal lines formed on the substrate perpendicularly to the gate lines, a plurality of thin film transistors, and pixel electrodes. Each of the gate lines is comprised of a gate wiring layer formed in a recess formed at a surface of the substrate.
Japanese Unexamined Patent Publication No. 10-239678 (A) has suggested a thin film transistor array substrate including an electrically insulating transparent substrate, a plurality of gate electrode lines formed on the substrate, a gate insulating film covering the gate electrode lines therewith, a plurality of source electrode lines intersecting with the gate electrode lines with the gate insulating film being sandwiched therebetween, a thin film transistor formed at an intersection of each of the gate electrode lines and each of the source electrode lines, a pixel electrode comprised of an electrically conductive film and electrically connected to a drain electrode of the thin film transistor, a capacitance electrode facing the pixel electrode with the gate insulating film being sandwiched therebetween to thereby define a capacity, and a black matrix layer formed below the pixel electrode and each of the source electrode lines and above the substrate. The black matrix layer is composed of the same material as a material of which the gate electrode lines are composed. The black matrix layer is electrically connected to the source electrode lines through a contact hole formed throughout the gate insulating film.
Japanese Unexamined Patent Publication No. 11-119253 (A) has suggested an active matrix type liquid crystal display device including a substrate, a plurality of scanning lines formed on the substrate, a plurality of signal lines formed on the substrate perpendicularly to the scanning lines, and a switching device fabricated in the vicinity of an intersection at which the scanning lines intersect with the signal lines. A pixel electrode is electrically connected to a drain electrode of the switching device. Each of the scanning lines is formed with apertures at opposite sides about each of the signal lines in the intersection. The apertures are covered with an electrically insulating film. Each of the apertures is covered with a part of the switching device or each of the signal lines.
However, the above-mentioned problems remain unsolved even in the above-mentioned Publications.
In view of the above-mentioned problems in the conventional liquid crystal display devices, it is an object of the present invention to provide an active matrix type liquid crystal display device which is capable of enhancing an aperture ratio without an increase in complexity in fabrication of the same.
It is also an object of the present invention to provide a method of fabricating such an active matrix type liquid crystal display device.
In one aspect of the present invention, there is provided an active matrix type liquid crystal display device including (a) a transparent substrate, (b) a plurality of scanning lines formed on the transparent substrate, (c) a plurality of signal lines formed on the transparent substrate perpendicularly to the scanning lines, (d) a transparent electrode arranged in a pixel area defined by the scanning and signal lines, and (e) a thin film transistor formed in association with the transparent electrode, and wherein the thin film transistor is formed in an area at which one of the scanning lines and one of the signal lines intersect with each other, and the thin film transistor includes source and drain regions both comprised of an electrically conductive film of which the signal lines are comprised.
For instance, the thin film transistor may be comprised of (e1) an island including a gate insulating film and a semiconductor layer both formed on each of the scanning lines in the area at which one of the scanning lines and one of the signal lines intersect with each other, (e2) a drain electrode comprised of a part of each of the signal lines which part extends across the island, and (e3) a source electrode comprised of an electrically conductive film of which each of the signal lines is comprised, and formed on the island with being spaced away from the drain electrode.
It is preferable that the active matrix type liquid crystal display device further includes a light-shielding layer surrounding the transparent electrode and being connected to the source electrode, the light-shielding layer being formed with a contact hole through which the source electrode is electrically connected to the transparent electrode.
It is preferable that the light-shielding layer has a portion having an increased width, the contact hole being formed at the portion. As an alternative, the light-shielding layer may have a uniform width, in which case, it is preferable that the contact hole has a rectangular cross-section having a longer side extending in a length-wise direction of the light-shielding layer.
It is preferable that each of the scanning lines has a portion having a decreased width in the area at which one of the scanning lines and one of the signal lines intersect with each other, the portion acting as a gate electrode of the thin film transistor.
It is preferable that the active matrix type liquid crystal display device further includes an island formed on each of the scanning lines at the portion having a decreased width, the island including a gate insulating film and a semiconductor layer both of which is greater in width than the portion.
It is preferable that the active matrix type liquid crystal display device further includes a first ohmic layer and a second ohmic layer both formed on the semiconductor layer such that the first and second ohmic layers are spaced away from each other, the first ohmic layer being formed as an extension of each of the signal lines and acting as a drain electrode of the thin film transistor, the second ohmic layer acting as a source electrode of the thin film transistor.
It is preferable that each of the signal lines has a portion having a decreased width in the area at which one of the scanning lines and one of the signal lines intersect with each other, the portion acting as a drain electrode of the thin film transistor.
In another aspect of the present invention, there is provided a method of fabricating an active matrix type liquid crystal display device, including the steps of (a) forming a plurality of scanning lines on a transparent substrate, (b) forming an island in an area at which one of the scanning lines and one of later mentioned signal lines intersect with each other, the island including a gate insulating film, a semiconductor layer and an ohmic layer all formed on each of the scanning lines in this order, (c) forming a plurality of signal lines extending perpendicularly to the scanning lines, each of the signal lines including a portion located on the island and acting as a drain electrode, (d) forming a source electrode on the island such that the source electrode faces and is spaced away from the drain electrode, (e) forming an interlayer insulating film entirely over a resultant resulted from the step (d), (f) forming a contact hole so that the source electrode partially appears, (g) forming a transparent electrode in a pixel area defined by the scanning and signal lines, and (g) electrically connecting the transparent electrode to the source electrode through the contact hole.
For instance, the steps (c) and (d) may be concurrently carried out.
It is preferable that each of the signal lines and the source electrode are concurrently formed in a common photolithography step.
It is preferable that the method further includes the step of forming a light-shielding layer which surrounds the transparent electrode and is connected to the source electrode, the light-shielding layer being formed with the contact hole.
It is preferable that the light-shielding layer is formed to have a portion having an increased width, the contact hole being formed at the portion.
It is preferable that the light-shielding layer is formed to have a uniform width, and the contact hole is formed to have a rectangular cross-section having a longer side extending in a length-wise direction of the light-shielding layer.
It is preferable that each of the scanning lines is formed to have a portion having a decreased width in the area at which one of the scanning lines and one of the signal lines intersect with each other, the portion acting as a gate electrode.
It is preferable that the island is formed on each of the scanning lines at the portion having a decreased width, the gate insulating film and the semiconductor layer both being greater in width than the portion.
It is preferable that the method further includes the step of forming a first ohmic layer and a second ohmic layer both on the semiconductor layer such that the first and second ohmic layers are spaced away from each other, the first ohmic layer being formed as an extension of each of the signal lines and acting as the drain electrode, the second ohmic layer acting as the source electrode.
It is preferable that each of the signal lines is formed to have a portion having a decreased width in the area at which one of the scanning lines and one of the signal lines intersect with each other, the portion acting as the drain electrode.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, an island of a thin film transistor is formed in an area at which a scanning line and a signal line intersect with each other. As a result, it is no longer necessary to arrange a thin film transistor in a pixel area defied by a scanning line and a signal line. Since only a contact hole is formed in a pixel area for electrically connecting a source electrode and a transparent electrode to each other, it would be possible to prevent an area of a pixel area, that is, an area of a transparent electrode from being reduced by a thin film transistor, ensuring enhancement of an aperture ratio.
In accordance with the present invention, it is possible to form both of a source electrode and a signal line as a drain electrode in a common step of carrying out photolithography to an electrically conductive film. Thus, it is possible to reduce the number of steps of carrying out photolithography by once in comparison with a conventional method of fabricating a liquid crystal display device in which source/drain electrodes and a signal line are formed in separate photolithography steps. As a result, a process of fabricating the liquid crystal display device can be simplified.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.